Primary pollutant in exhaust gas of motor vehicle is incompletely burned hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxide (NOx), wherein more than two thirds hydrocarbons are exhausted out during the cold-start phase of motor vehicle. In the cold-start phase, especially within about 100 seconds after the first start of an engine, the temperature of a vehicle exhaust catalytic converter still doesn't come to the optimal working temperature and can not provide a good purification effect, so that it is important to reduce the emission of hydrocarbons during the cold-start phase of motor vehicle, in order to control the atmosphere pollution.
Two methods can be used to treat the emission of hydrocarbons during the cold-start phase of motor vehicle, wherein one method is to increase the temperature of the catalytic converter for speeding the activation of catalyst mainly by means of delay of sparking, thermal insulation of exhaust-pipe, electrically heating of catalytic converter, combustion heating of catalytic converter and etc.; and the other method is to install a hydrocarbon absorber prior to the catalytic converter for absorbing hydrocarbons when the temperature of the exhaust system is relatively low, in order to reduce the emission of hydrocarbons. When the temperature of the system raises, the absorbed hydrocarbons may desorb and enter the catalytic converter to be oxidized. However, the foregoing technology is too complicated, and will correspondingly increase the cost. Therefore, it becomes a research hotspot to install a close coupled catalyst having a position close to an exhaust outlet of an engine for being widely applied to the field of controlling exhaust gas of motor vehicle.
The close coupled catalyst is generally used with a chassis three-way catalyst. As the close coupled catalyst is closer to the exhaust outlet of the engine, it is also called an upstream catalyst, a pre-catalyst. Because the temperature of the cold-start phase is lower, the close coupled catalyst must have the higher low-temperature activation. However, after the engine keeps stably working, the close coupled catalyst will directly expose under high-temperature exhaust gas (more than 1000° C.), so that the close coupled catalyst must simultaneously have a higher high-temperature stability to undergo the temperature exhaust of 1100° C. or more than 1100° C.
U.S. Patent Publication No. 2001/0036432 discloses a close coupled catalyst prepared by using palladium nitrate, γ-Al2O3 having a surface area of about 160 m2/g, zirconium acetate, lanthanum nitrate, zirconium nitrate, neodymium nitrate, barium nitrate, strontium nitrate and cordierite, wherein the catalyst includes 100 g/ft3 of Pd, 1.23 g/in3 of Al2O3, 0.19 g/in3 of La2O3, 0.1 g/in3 of ZrO2, 0.1 g/in3 of SrO, and 0.16 g/in3 of Nd2O3.
Chinese Patent Application No. 200610020144.9 discloses a close coupled catalyst which comprises at least three kinds of alumina with different crystal phases as a catalyst carrier, wherein Pd is used as an active component; at least one of rare-earth oxides including La2O3, Nd2O3, Y2O3, and Sm2O3, at least one of alkaline earth oxides including SrO, BaO and CaO, and ZrO2 are used. Hydrocarbon light-off temperature is below 250° C. over the catalyst aged at 1050° C. in the wet air.
U.S. Pat. No. 6,254,842 discloses a close coupled catalyst which is aged in 10% steam-air at 950° C. for 12 hours. Using this catalyst, the conversion temperature for HC, CO and NOx is 252° C., 228° C. and 213° C., respectively.
U.S. Pat. No. 6,602,822 discloses that the light-off temperature of a close coupled catalyst for the purification of hydrocarbon can be lowered by suitably lowering the thickness of the coating. U.S. Pat. No. 5,878,567 discloses a closed coupled catalyst prepared by highly loaded palladium or tri-metal catalytic elements.
Although the foregoing technologies can low the exhaust gas during the cold-start phase, the cost of treating the exhaust gas of motor vehicle will be increased. U.S. Pat. No. 6,254,842 discloses that the closed coupled catalyst can provide the function of three-way catalyst, so as to partially replace the three-way catalyst for lowering the cost.
In order to enhance the performance of the close coupled catalyst, the preparation method of the catalyst and the structure design thereof are substantially researched, and a close coupled catalyst having dual layer structures is developed.
Chinese Patent No. 1197686A discloses a preparation of catalyst for exhaust gas purification and with high activity for the hydrocarbon conversion, wherein the catalyst consists of platinum metals as catalytically active components and two functional coatings stacked on a carrier. A first functional coating comprises a mixture of a first catalyst component and at least one type of zeolite, and the second functional coating comprises a mixture of a second catalyst component and at least one type of zeolite. The platinum metal is at least palladium and rhodium which are separately from each other and contained in each of the two catalysts.
U.S. Pat. No. 5,057,483 discloses a three-way catalyst located inside an internal combustion engine, wherein the catalyst has two coatings. A first coating includes alumina, platinum and cerium oxide, and iron oxide and nickel oxide can be added for the suppression of hydrogen sulfide emissions. Further, baria and zirconia can be added to enhance the stability of the first coating. The second coating is disposed on the first coating, and includes rare earth oxide-zirconia, rhodium, alumina and a noble metal except for rhodium.
U.S. Pat. No. 6,875,725 discloses a closed coupled catalyst which is also a dual-layer catalyst, wherein a first coating includes aluminum oxide, barium oxide and palladium, and a second coating includes aluminum oxide with 3 wt % lanthanum oxide, γ-aluminum oxide, praseodymium oxide, cerium/zirconium mixed oxides, palladium, and rhodium.
U.S. Patent Publication No. 2003/0039597 invents a closed coupled catalyst for the removal of pollutants during the cold-start phase of motor vehicle. The catalyst has two sections, wherein an upstream section includes one platinum-group element and a SOx sorbent (selected from the group consisting of oxides and mixed oxides of barium, lanthanum, magnesium, manganese, neodymium, praseodymium, and strontium), while a downstream section comprises another platinum-group element and a NOx sorbent having NOx absorption function.
As described above, it is an object for researchers to develop a closed coupled catalyst having better high-temperature stability, higher activity and lower selling price.